I can't remember specifically about high blood pressure medicine, but I know some people continue taking their meds such as anti-depressants, birth control, etc. during the MC. Is one of your Master-Cleanse goals to get off the high blood pressure med? If so, maybe you can ask your doctor (if you trust him or her) what is the worst thing that could happen if you stopped taking the med.
This question comes up a lot and I don't think I've ever seen a definitive answer on it.